[Elecraft] Re: RF Coupleing

[Eric Swartz - WA6HHQ, Elecraft]

Ron - Unfortunately theory does not always match reality. (i.e. there 
are more variables than the person applying the theory is aware of..) In 
this case we tried coax and it had no impact on improving isolation.  
The predominant leakage path is inside the ant switching relay. Please 
do not post that though as Wayne will make the official statement when 
he has time.

e

Ron D'Eau Claire wrote:
> Lennart's comments fit my experience. There isn't a "ground loop" formed by
> using coax and it does eliminate much of the RF currents floating about at
> random inside the rig.
>
> With the RF current flowing at the center pin connected to coax, the RF
> energy must propagate along the *inside* of the coax, with RF currents
> flowing on the outside of the center conductor and on the *inside* surface
> of the shield. There is only one significant path for the RF: through the
> coax. 
>
> Since the center conductor, carrying 1/2 of the RF current, is electrically
> isolated from the rest of the rig, there is nothing to encourage RF current
> to flow from the connector outer conductor through the metal cabinet to some
> other terminal. As noted, RF isn't DC or low-frequency AC. Indeed, it's
> useful to consider a wire (or coax) carrying RF as a sort of "wave-guide";
> not the classic stuff used at microwaves, but nevertheless a guide that
> directs the RF energy. In that respect, the RF energy flows through the
> space between the conductors while inducing RF currents along the surfaces
> of those conductors. In coax, the RF energy flows through the dielectric
> between the center and shield. That's why dielectric has such a huge effect
> on the electrical length of a piece of coax. It retards the RF wave
> propagation. 
>
> If a bare wire is used, the RF will flow between it and some convenient
> conductor connected to the shielded side of the conductor, such as the
> cabinet, pc board ground, etc. That produces an RF field that fills the
> space between them which might include lots of other circuits in which the
> RF field induces currents. Those are currents that can cause mischief if
> they get into an RF-sensitive circuit. 
>
> In any design where I want to isolate the RF I use coaxial cable on all
> internal RF connections. Generally it's "overkill" but that's the luxury of
> building a one-only homebrew project in which a few extra dollars in parts
> is insignificant. That's not the case in something that must be manufactured
> at a competitive price and at a profit. That requires engineering expertise
> that can evaluate exactly where additional shielding is needed and how much.
>
> As I often note, engineering is all about making compromises. Great
> engineering is about making great compromises. 
>
> Yes, a "hood" on back of a female SO239 might be good, but I've found little
> need for one at HF. It might be needed in this case if extreme isolation is
> wanted, but I'd be surprised if that were the case.
>
> Ron AC7AC
>
>
> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net
> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Lennart Michaëlsson
>
> Vic,
> I would not go for that solution because the insulated part of the coax
> inside the cabinet would reradiate anything coming on the coax from the
> outside. Best solution is short piece of coax grounded on the inside of
> cabinet AND at the antenna tuner. That is my opinion, humbly yours Len
> SM7BIC
>
> That's certainly part of the problem. But just adding a ground wire might
> create a ground loop. I guess the best way to do it would be to insulate the
> SO239's from the chassis, and use hoods on them with coax!
> --
> 73,
> Vic, K2VCO
> Fresno CA
> http://www.qsl.net/k2vco _______________________________________________
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